Laravel中的多重关系(pivot)

class User extends Model
{
    public function roles()
    {
        return $this->belongsToMany(Role::class);
    }
}

class Role extends Model
{
    public function users()
    {
        return $this->belongsToMany(User::class);
    }
}

Schema::create('role_user', function (Blueprint $table) {
    $table->unsignedBigInteger('user_id');
    $table->unsignedBigInteger('role_id');
    $table->timestamps();

    $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
    $table->foreign('role_id')->references('id')->on('roles')->onDelete('cascade');
});

$user = User::find(1);
$roles = $user->roles;

$user = User::find(1);
$user->roles()->attach($roleId);

$user = User::find(1);
$user->roles()->detach($roleId);